The present invention is related to a method for controlling a wind energy plant, with a nacelle disposed on a tower and with a rotor with at least one rotor blade, the blade adjustment angle of which can be adjusted by means of a blade adjustment equipment, in which the nacelle is rotated around the tower axis.
Wind energy plants in which the blade adjustment angle (also called pitch angle) is adjusted, i.e. the angle of the rotor blade with respect to the rotor plane, are known since a long time. By means of a blade adjustment equipment, the blade adjustment angle can be adjusted depending on the wind velocity acting on the blade (also called pitch adjustment). Thus, at a blade adjustment angle of 90°, there is a minimum power conversion of the wind into mechanical energy, whereas at 0°, maximum power conversion takes place. Such adjustment equipments are provided with electric or hydraulic drives in many cases. In particular, an individual adjustment equipment may be provided for each rotor blade of a wind energy plant.
Plants are also known, in which when exceeding a maximum wind velocity, a stall on the blade, and by doing so a drastic reduction of the power conversion, can be created by means of a blade adjustment equipment (“active stall”). Plants described above with a pitch control and plants with an active stall control are described in Erich Hau: “Windkraftanlagen”, 3th edition, Springer Verlag, p. 89 ff, the entire contents of which is incorporated herein by reference.
Furthermore, it is known to rotate the nacelle of wind energy plants around the tower axis via an azimuth drive, in order to align the wind energy plant corresponding to the actual wind direction, and thus to optimize the yield. For this purpose, the nacelle is rotated such that it points into the wind with its rotor. In the state of the art, it is also known to avoid a damage of the plant due to excessive load when too high wind velocities occur, by performing a rotation of the nacelle abround the tower axis.
For example, in DE 197 17 059 C1, the entire contents of which is incorporated herein by reference, it is proposed for an active stall plant to adjust the blade pitch angles of the rotor blades such that the blades occupy an angle of −90° with respect to the rotor plane, and to turn the nacelle abround the tower axis about 180° to the lee side of the tower thereafter. From EP 0 709 571 A2, the entire contents of which is incorporated herein by reference, it is furthermore known to adjust the blade adjustment angles of the rotor blades such that the rotor rotates automatically around the tower axis into a load-depleted parking position. Finally, in DE 100 58 076 A1, the entire contents of which is incorporated herein by reference, it is proposed to adjust the blade adjustment angles of the rotor blades in a first wind velocity such that they stand in the flag position or generate a stall, respectively. Further, it is proposed to rotate the rotor about 180° around the tower axis towards the lee side when there is a second, higher wind velocity.
However, the known procedures are only possible when the blade adjustment equipment works without problems. When in contrary the blade adjustment equipment fails, the proposed methods can no more be executed in a reliable way. In this case, an exceeding of the maximum permitted rotational speed of the rotor can take place in a strong wind load. Such an excessive rotational speed cannot even be prevented by the rotor brake provided in wind energy plants. This is because the braking power of this brake is not dimensioned to diminish the rotor's rotational speed against strong attacking wind. There is the danger of damage of the rotor brake. In addition, at a malfunction of the blade adjustment equipment, the load by the attacking wind on the plant and on the rotor and its blades in particular is still too high in part, even at the parking position proposed in the state of the art.
Based on the explained state of the art, the present invention has the objective to indicate a method of the initially mentioned type by which a reduction of the loads acting on the plant is provided in a malfunction of the blade adjustment equipment.